1 Field of the Invention
The present invention relates to a display device and a production method thereof. More specifically, the present invention relates to a display device such as a liquid crystal display device, which provides color display using colored layers formed by a method of applying a droplet, such as an inkjet method, and to a production method of such a display device.
2 Description of the Related Art
In display devices such as a liquid crystal display device, various contrivances have been introduced to prevent a reduction in contrast ratio, a change in color tone, darkening of display gradation, and the like when a screen is viewed in an oblique direction. Further, the following problem has been recently pointed out. A difference in gradation level of display luminance between when the screen is viewed in the front direction and when it is viewed in an oblique direction causes a difference in display state depending on an observation direction, and further, such a difference also influences on display of images such as a photograph or TV screen display. Improvements in such a point have been desired. Under such a circumstance, for example, a method of enhancing display qualities by dividing each pixel formed on a thin film transistor (TFT) array substrate into plural regions, and applying a voltage to each region, thereby adjusting a gradation level of display luminance (for example, refer to Patent Document 1).
A color filter (hereinafter, also referred to as a CF) used in the liquid crystal display device and the like generally has a structure in which colored layers (filters) of three primary colors of light, i.e., red (R), green (G), and blue (B), are formed on a transparent substrate. The following methods are commonly used as a production method of the CF substrate: a pigment dispersion method of applying a color resist on a transparent substrate, exposing and developing the resist, thereby pattern-forming a colored layer; a dyeing method of applying a dyeing base material on a transparent substrate, dyeing a pattern formed by exposing and developing the material to form a colored layer; an electrodeposition method of pattern-forming a transparent electrode on a transparent substrate, applying a current to the transparent electrode in an electrolyte of each color, thereby forming a colored layer by electrodeposition; and a printing method of printing each color on a transparent substrate, thereby forming a colored layer. However, according to the pigment dispersion method and the dyeing method, the respective steps of coating, exposure, and development need to be repeatedly performed for each color. Therefore, the processes are difficult to simplify. In addition, spin coating in the application step increases loss of materials. According to the electrodeposition method, the pattern shape is limited. Further, a high-definition pattern is difficult to form by the printing method. Therefore, an ink-jet method has attracted attention as a new alternative method of forming a colored layer.
According to the ink-jet method, an ink-jet apparatus equipped with an ink-jet head having a plurality of nozzles is used. While the ink-jet head is moved over a transparent substrate, colored materials of red (R), green (G), and blue (B) are injected to directly form a colored layer pattern. This method needs no exposure and development steps, which can simplify the processes. Further, the use amount of a material for the colored layer can be decreased. As a result, costs can be reduced.
As one method of preparing a CF substrate by an ink-jet method, a method in which a bank is formed on a glass substrate, and ink is injected into a region surrounded by the bank to form a colored layer is disclosed (for example, refer to Patent Document 2). A black matrix (hereinafter, also referred to as a BM) that is formed on the substrate can be used also as a bank. Ink is injected into a region surrounded by the BM and dried to form a colored layer.
The applied ink flows on the substrate, which possibly causes a variation in thickness of the formed colored layer. This ink flow is remarkably observed in a stripe bank pattern, but hardly observed in a dot bank pattern. If the CF substrate is prepared by an ink-jet method, inks might be mixed between adjacent pixels to cause a color mixing defect. Therefore, in order to easily correct the color mixing defect, the dot bank pattern is also better than the stripe bank pattern.
If a pixel division configuration that has been recently adopted to a liquid crystal TV and the like as a wide viewing angle technology is employed in combination with the method of producing a CF substrate by an ink-jet method, the bank structure might hinder drawing of a colored layer pattern that is formed with an ink-jet apparatus. This is mentioned in more detail below with reference to FIG. 3. FIG. 3 is a planar view schematically showing each substrate of a display device in accordance with the pixel division configuration. FIG. 3(a) shows a TFT substrate. FIG. 3(b) shows a CF substrate in which a CF is divided along a storage capacitor wiring. FIG. 3(c) shows a CF substrate in which a CF is divided along a scanning line.
As shown in FIG. 3(a), a plurality of signal lines 15 and a plurality of scanning lines 16 are arranged to be perpendicular to each other on the TFT substrate. In addition, a storage capacitor wiring 17 is arranged between the scanning lines 16 to be perpendicular to the signal lines 15 and parallel to the scanning lines 16. At each intersection of the signal lines 15 and the scanning lines 16, two TFTs 18a and 18b that are switching elements are arranged with the scanning line 16 therebetween. These two TFTs 18a and 18b are arranged in a pixel 14 that is an image display unit controlled by the signal line 15 and the scanning line 16 to divide the pixel 14 into two sub-pixels 14a and 14b and control them.
Two configurations shown in FIGS. 3(b) and 3(c) are mentioned as a configuration of the CF substrate that makes a pair with the TFT substrate shown in FIG. 3(a). First, in FIG. 3(b), a bank 31 is arranged in regions overlapping with the signal line 15, the storage capacitor wiring 17, and the TFTs 18a and 18b. The TFTs 18a and 18b are arranged with the scanning line 16 therebetween. Therefore, the bank 31 has a TFT shielding part 30 that expands from a region other than the corner. When ink is injected into a coloring region by an ink-jet method, in order to prevent color mixing between adjacent coloring regions or prevent the ink from being spread on the bank, a margin of ink injection needs to be secured by setting a position where the ink is injected to be separated from the bank 31 by a certain distance (30 μm or more if a common ink-jet apparatus and common ink are used). Accordingly, if an ink-injected region 23 is formed in such a way that ink is not injected on the TFT shielding part 30, the ink-injected region 23 is too small. In such a case, a moving speed of the ink-jet head needs to be decreased or the ink needs to be injected several times, for injecting droplets in a required amount. As a result, production efficiency of the colored layer 32 is reduced. Even if droplets in a required amount are injected, the ink is difficult to spread into the vicinity a hem of the TFT shielding part 30, and the ink is not uniformly spread. As a result, a phenomenon in which the absence of the colored layer 32 or insufficient thickness of the colored layer 32 causes insufficient coloring of transmissive light, a so-called void phenomenon might be generated near the TFT shielding part 30.
In FIG. 3(c), a bank 41 is arranged in regions overlapping with the signal line 15, the scanning line 16, and the TFTs 18a and 18b . That is, the bank 41 is arranged not in the region overlapping with the storage capacitor wiring 17 but in the region overlapping with the scanning line 16. According to such a structure, a TFT shielding part 40 is arranged at a corner of the coloring region, and therefore, generation of the above-mentioned void phenomenon can be suppressed. However, a pixel formation period 103 in the TFT substrate is different from a colored layer formation period 104 in the CF substrate by one sub-pixel. Therefore, an area of a coloring region 52 positioned at an end of the display region is half that of a coloring region 42 in the display region. Even in the coloring region 52, the above-mentioned margin of ink injection needs to be provided, and therefore, a region where ink can be actually injected is further decreased. Therefore, an area of a region 43 where ink can be injected in the coloring region 52 is half or smaller than that of a region 33 where ink can be injected in the coloring region 42 in the display region. Therefore, it takes a long time to complete the ink injection, which leads to a reduction in production efficiency of the colored layers. Accordingly, there is room for improvement in this point.
[Patent Document 1]
    Japanese Kokai Publication No. 2004-62146[Patent Document 2]    Japanese Kokai Publication No. Hei-07-318723